Static Electricity
Introduction Even if you have never heard of static electric, it is certain that you have come into contact with it. The properties and affects of static electricity become more obvious when it is seen in action. For example many kid enjoy rubbing balloons on their head, why? Because the balloons temporarily stick! This phenomenon can be attributed to static elecity and charge. Another example of a situation everyone has been through, a static shock. This event can also be attributed to static electricity! So what is static electricity? Static Electricity is defined as an electrical charge caused by an imbalance of electrons on the surface of a material. Put in simple terms, static electricity is when electrons (not protons) are moved, usually by rubbing or brushing. There is also one more way to define static electricity, by defining and comparing its opposite. Current electricity is the opposite of static electricity, the difference between the two is static is when electrons are moved, as stated above, and current is when electrons flow inside a conductor. So just remember, static electricity is a build up of charge, positive or negative, in one place. If you had some trouble understanding this introduction please look to the background/fundamentals section, if not continue on the learn more about static electricity and its applications. ---- Background/Fundamentals Before delving into static electricity some important terms must be defined and understood: Protons A proton is a positive (+1) charge its specific charge is a positive elementary charge or 1.6 x 10 -19 Electrons An electron is a negative (-1) charge its specific charge is a negative elementary charge or –1.6 x 10 -19 Charge An object can have two electrical charges either positive, or negative. A positive charge is a deficiency of electrons and negative charge is an excess of electrons. So electrons are negative, protons are positive, and neutrons are neutral. It is also important to note that only electrons are transferred from object to object, an example of this transfer is friction. For further properties of charges please look to the diagrams below of repelling and attracting charges. Now that the definitions of protons, electrons and charge have been established, the formula for charge can be introduced. The formula for charge is '''Q = ne'''. In the equation Q stands for charge of an object in coulombs, n is the number of elementary charges, and e is the elementary charge (found on NYS Physics Reference Tables, to be 1.60 x 10 -19 . See this equation in action: A person has received a charge of 8 coulombs how many elementary charges does the person have? *'''Q=ne''' First setup the equation **'''8 C= n(1.60 x 10 -19 C)''' Next plug in the given 8 C and plug in elementary charge e (found on NYS Physics Reference Table) **'''5.0 x 10 19 C = n''' Now divde the given Q by the elementary charge e. Now we know that there are 5.0 x 10 19 elementary charges! ---- How Are Objects Charged? Before learning how to charge and object, it is important to understand the objects used in charging an object. Usually in a situation in which a object is going to be charged both a neutral object and an object that posses a charge are needed. It is very important to note that the charged object confines a positive or negative charge. This important requirement is actually given a name, an insulator. Insulators are materials that hold the charge they are given. So if a person wanted to charge an object negatively, the process of charging by conduction could be used. First a negatively charged insulator, possibly a glass rod, would slowly come into contact with the neutrally charged object. As the rod gets closer, the positive and negative charges will start to detach and the positive charges will be attracted to the negative charges in the rod. When the rod comes into full contact, an exchange of electrons will occur, and the once neutrally charged object will have an excess of electrons making the object charge negative. This process is depicted in the figure to the right. There are two other ways to charge an object, redistrubtion and grounding. Redistrubtion is a method similair to charging by conduction exept the two objects never touch, as a result the charging is only temporary. Because the two objects never touch there is essentially no transfer, the only event that takes place is the charges temporarley seperating in the neutrally charged object. Grounding, also known as induction, is a method quite different to the previous two. In grounding the objects are moved closed together so the charges seperate, as in redistrubtion, but then the neutral object is 'grounded' which means being attached to the ground, or any large neutral object such as the Earth, after being grounded the electrons will leave the object and the objecct will have defeiency of electrons resulting in a postive charge. How to Detect Charge So the process of charging an object has been established, but how is this charge detected? Charge is not a visible quantity, so scientist can not measure it using the naked eye– rather an instrument called and electrocscope is what detects charge. A diagram of an electroscope is provided to the right. The plate at the top of the electroscope is what detects the charge. The line moving, is called th leaf. In the rested upright position there is no charge detected. In the rotated postion a charge is detected. An electroscope alone will not reveal the charge of an object. An electroscope in conjunction with a known charge will produce results which are readable interms of positve and negative rather than charge or no charge. Measuring the Forces Acting on Charges We now know the properties of charges, how to charge an object, and how charges are detected, but we still do not know how to measure charges interactions. When two charges interact, there are two possiblites the charges can either attract or repel (as depicted in the fundementals section).In both of these possibities, the charges feel eitther a retractive or repulsive force– we know that this force exists, but how can we measure it? This same question was asked by Charles-Augustin de Coulomb in 1784. Coulomb was a great French phyisist who was responsible for discovering a relationship between the force of two charges and the square of its distance. He compiled this relationship into an equation called Coulomb's Law. The equation is http://upload.wikimedia.org/math/d/4/0/d400ebe3e561448fd8fff5c6608599d1.png. In this equation F stands for the total force exerted, q 1 is the charge on one of the bodies, q 2 is the charge on the other body, k is the electrostatic constant (Found on the NYS Physics Reference Table) which is 9.0 x 10 9 n x m 2 / C 2 , and r is the distance between the two bodies. Electric Fields To understand the concept of electric fields an understanding of electric force and charge is a prerequsite. An electric field is created when an electric force, the F constant in Coulomb's law, is exerted on to a charged particle. Real World Applications Practice Problems References Resources